TY - JOUR A1 - Niemz, Peter A1 - Cesca, Simone A1 - Heimann, Sebastian A1 - Grigoli, Francesco A1 - von Specht, Sebastian A1 - Hammer, Conny A1 - Zang, Arno A1 - Dahm, Torsten T1 - Full-waveform-based characterization of acoustic emission activity in a mine-scale experiment BT - a comparison of conventional and advanced hydraulic fracturing schemes JF - Geophysical journal international / the Royal Astronomical Society, the Deutsche Geophysikalische Gesellschaft and the European Geophysical Society N2 - Understanding fracturing processes and the hydromechanical relation to induced seismicity is a key question for enhanced geothermal systems (EGS). Commonly massive fluid injection, predominately causing hydroshearing, are used in large-scale EGS but also hydraulic fracturing approaches were discussed. To evaluate the applicability of hydraulic fracturing techniques in EGS, six in situ, multistage hydraulic fracturing experiments with three different injection schemes were performed under controlled conditions in crystalline rock at the Aspo Hard Rock Laboratory (Sweden). During the experiments the near-field ground motion was continuously recorded by 11 piezoelectric borehole sensors with a sampling rate of 1 MHz. The sensor network covered a volume of 30x30x30 m around a horizontal, 28-m-long injection borehole at a depth of 410 m. To extract and characterize massive, induced, high-frequency acoustic emission (AE) activity from continuous recordings, a semi-automated workflow was developed relying on full waveform based detection, classification and location procedures. The approach extended the AE catalogue from 196 triggered events in previous studies to more than 19600 located AEs. The enhanced catalogue, for the first time, allows a detailed analysis of induced seismicity during single hydraulic fracturing experiments, including the individual fracturing stages and the comparison between injection schemes. Beside the detailed study of the spatio-temporal patterns, event clusters and the growth of seismic clouds, we estimate relative magnitudes and b-values of AEs for conventional, cyclic progressive and dynamic pulse injection schemes, the latter two being fatigue hydraulic fracturing techniques. While the conventional fracturing leads to AE patterns clustered in planar regions, indicating the generation of a single main fracture plane, the cyclic progressive injection scheme results in a more diffuse, cloud-like AE distribution, indicating the activation of a more complex fracture network. For a given amount of hydraulic energy (pressure multiplied by injected volume) pumped into the system, the cyclic progressive scheme is characterized by a lower rate of seismicity, lower maximum magnitudes and significantly larger b-values, implying an increased number of small events relative to the large ones. To our knowledge, this is the first direct comparison of high resolution seismicity in a mine-scale experiment induced by different hydraulic fracturing schemes. KW - Fracture and flow KW - Spatial analysis KW - Statistical methods KW - Time-series analysis KW - Induced seismicity Y1 - 2020 U6 - https://doi.org/10.1093/gji/ggaa127 SN - 0955-419X SN - 1365-246X VL - 222 IS - 1 SP - 189 EP - 206 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Platz, Anna A1 - Weckmann, Ute T1 - An automated new pre-selection tool for noisy Magnetotelluric data using the Mahalanobis distance and magnetic field constraints JF - Geophysical journal international N2 - In Magnetotellurics (MT) natural electromagnetic field variations are recorded to study the electrical conductivity structure of the subsurface. Thereby long time-series of electromagnetic data are subdivided into smaller segments, which are Fourier transformed and typically averaged in a statistically robust manner to obtain MT transfer functions. Unfortunately, nowadays the presence of man-made electromagnetic noise sources often deteriorates a significant fraction of the recorded time-series by overprinting the desired natural field variations. Available approaches to obtain undisturbed and high quality MT results include, for example robust statistics, remote reference or multi-station analyses which aim at the removal of outliers or uncorrelated noise. However, we have observed that intermittent noise often affects a certain time span resulting in a second cluster of transfer functions in addition to the expected true MT distribution. In this paper, we present a novel criterion for the detection and pre-selection of EM noise in form of outliers or additional clusters based on a distance measure of each data segment with regard to the centre of the data distribution. For this purpose, we utilize the Mahalanobis distance (MD) which computes the distance between two multivariate points considering the covariance matrix of the data that quantifies the shape and the size of multivariate data distributions. As the MD considers the covariance matrix, it corrects not only for different variances but also for any correlation between the data. The computation of both, the mean value and covariance matrix, is susceptible to ouliers (e.g. noise) and requires a statistically robust estimation. We tested several robust estimators, for example median absolute deviation or minimum covariance determinant algorithm and finally implemented an automatic criterion using a deterministic minimum covariance determinant algorithm. We will present results using MT data from various field experiments all over the world, which illustrate successfull data improvement. This approach is able to remove scattered data points as well as to reject complete data cluster originating from noise sources. However, like all purely statistical algorithms the criterion is limited to cases where the majority of the recorded data is well-behaved, that is noise content is below 50 per cent. If the majority of data points originates from noise sources, the new criterion will fail if used in an automatic way. In these cases, additional input by the user either manually or in an automated fashion can be utilized. We therefore suggest to use an add-on criterion to back the MD selection and subsequent robust stacking in form of a physically motivated constraint based on the magnetic incidence direction. This property indicates whether the magnetic field originates from various sources in the far field or from a strong and well defined source in the near field. KW - Magnetotellurics KW - Statistical methods KW - Time-series analysis Y1 - 2019 U6 - https://doi.org/10.1093/gji/ggz197 SN - 0956-540X SN - 1365-246X VL - 218 IS - 3 SP - 1853 EP - 1872 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Pick, Leonie A1 - Korte, Monika T1 - An annual proxy for the geomagnetic signal of magnetospheric currents on Earth based on observatory data from 1900–2010 JF - Geophysical Journal International N2 - We introduce the Annual Magnetospheric Currents index as long-term proxy for the geomagnetic signal of magnetospheric currents on Earth valid within the time span 1900–2010. Similar to the widely used disturbance storm time and ‘Ring Current’ indices, it is based on geomagnetic observatory data, but provides a realistic absolute level and uncertainty estimates. Crucial aspects to this end are the revision of observatory crustal biases as well as the implementation of a Bayesian inversion accounting for uncertainties in the main field estimate, both required for the index derivation. The observatory choice is based on a minimization of index variance during a reference period spanning 1960–2010. The new index is capable of correcting observatory time series from large-scale external signals in a user-friendly manner. At present the index is only available as annual mean values. An extension to hourly values for the same time span is in progress. KW - Magnetic field variations through time KW - Satellite magnetics KW - Inverse theory KW - Statistical methods KW - Time-series analysis Y1 - 2017 U6 - https://doi.org/10.1093/gji/ggx367 SN - 1365-246X SN - 0956-540X VL - 211 IS - 2 SP - 1223 EP - 1236 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Strader, Anne A1 - Schneider, Max A1 - Schorlemmer, Danijel T1 - Prospective and retrospective evaluation of five-year earthquake forecast models for California JF - Geophysical journal international KW - Probabilistic forecasting KW - Statistical methods KW - Earthquake interaction KW - fore-casting KW - and prediction KW - Statistical seismology Y1 - 2017 U6 - https://doi.org/10.1093/gji/ggx268 SN - 0956-540X SN - 1365-246X VL - 211 SP - 239 EP - 251 PB - Oxford Univ. Press CY - Oxford ER -